A device of the above-mentioned type has become known from U.S. Pat. No. 3,420,232. Saturated anesthetic vapor from an evaporator chamber is mixed with the anesthetic gas via a dispensing means in the anesthetic evaporator operating according to the bypass principle in order to provide a predetermined concentration of anesthetic in the anesthetic gas. The anesthetic evaporator is provided with a shut-off valve, which is designed to block the gas flow from the evaporator chamber in a first switching position, so that the anesthetic gas flows via a bypass line from a gas inlet to a gas outlet. In a second switching position, the gas flow is released via the evaporator chamber and anesthetic vapor can be mixed with the anesthetic gas by means of the dispensing device. The shut-off valve comprises a lower part, which is provided with gas ducts and is part of the evaporator housing, and a rotatably movable upper part, which is attached thereto and has kidney-shaped gas ducts. Depending on the angular position of the upper part, the kidney-shaped gas ducts of the upper part connect corresponding gas ducts in the lower part, so that a gas flow is released via the shut-off valve or the gas ducts in the lower part are closed.
The upper part has a carrier pin, which is connected with the anesthetic concentration setting member. The shut-off valve is closed in the zero position of the setting member and the anesthetic gas flows via the bypass line directly from the gas inlet to the gas outlet. If the setting member is set from the zero position to a certain anesthetic concentration, the shut-off valve opens above the carrier pin and the gas flow from the evaporator chamber is released.
A certain system pressure must be maintained within the anesthetic tank in case of anesthetics with low boiling point in order to prevent the anesthetic from boiling. A pressure control circuit with a differential pressure sensor and a proportional valve is usually used for the dispensing branch in such evaporators. The carrier gas, which is to be enriched with anesthetic vapor, now flows from a gas inlet to a gas outlet over a first throttle set in a fixed manner, a so-called bypass gap. The anesthetic, which is in the form of a vapor, is mixed with the carrier gas behind the first throttle. The anesthetic evaporated in an evaporator chamber is sent for this purpose via a proportional valve and a second throttle adjustable by the user by means of a setting member. The differential pressure is detected with the differential pressure pick-up upstream of the first throttle and the second throttle. The pressure in front of the second throttle is set with the proportional valve such that the pressure difference equals, on average, zero and an anesthetic concentration that is independent from the gas flow of the carrier gas can thus be set.
An anesthetic evaporator of the type mentioned has become known from EP 469 797 B2. The differential pressure pick-up, which is needed for the pressure control, must be calibrated at regular intervals in order to compensate drift effects. Provisions are made for this purpose for the pneumatic connections of the differential pressure pick-up to be connected with a common pressure source via a separate switchover means, and the common pressure source may also be at atmospheric pressure.